Thermionic Generators
Thermionic energy conversion (TEC) is a technique that allows for the efficient conversion of thermal energy directly into electrical energy. Thermionic energy conversion is the direct production of electric power from heat by thermionic electron emission. Essentially it is the use of electron vapor as the working fluid in a power-producing cycle. A thermionic converter consists of a hot emitter electrode from which electrons are vaporized by thermionic emission and a colder collector electrode into which they are condensed after conduction through the inter-electrode plasma. The resulting current, typically several amperes per square centimeter of emitter surface, delivers electrical power to a load at a typical potential difference measured in volts. The thermal efficiency depends on the emitter temperature and mode of operation.
TEC is based on the physical principal of thermionic emission which describes the thermal emission of electrons from a heated cathode. As a cathode is heated above zero Kelvin, it can be predicted, based on Fermi-Dirac statistics that some of the cathode's electrons will have energies equal to or greater than the cathode's work function. The work function is the energy required for an electron to be emitted into the vacuum. The function of the cathode in such a heat engine is to act as an “electron boiler”, while the anode functions as the electron condenser. These two components result in electric pressure (i.e., voltage), difference which produces work. The difference between the heat of vaporization of the electrons from the emitter and the heat of condensation of the electrons into the collector directly equals the amount of electrical work produced per electron.
Thermionic generators are devices that can directly convert thermal energy to electrical energy. The most basic configuration of a thermionic generator consists of three primary components: a cathode, an anode, and the necessary electrical connections between the two. The cathode and anode are separated by either a vacuum or a certain gaseous environment. As thermal energy is imparted to the cathode, electrons with sufficient energy will be ejected by a process known as thermionic emission and will traverse the inter-electrode gap to be collected at the anode. The electrons then drive an electrical load as they are cycled back to the cathode through the electrical connections.
The performance of a thermionic energy converter, (“TEC”), device is highly influenced by the cathode's ability to emit electrons when heated. This thermionic emission process is follows a relation known as the Richardson equation, defined as:J=AT2e−φ/kT where J is the thermionic emission current density in Amps per square centimeter; A is the cathode's Richardson constant in units of Amps per square centimeter per kelvin; T is the cathode temperature in Kelvin; φ is the cathode work function in electron volts; and K is the Boltzmann Constant in electron volts per Kelvin. It thus follows from the Richardson equation that the emission current is exponentially dependent on the samples work function which is defined as the energy barrier that electrons must overcome in order to be emitted from a cathode. Thus, materials with lower work functions can emit more electrons at the same temperature as materials with higher work functions assuming identical pre-exponential Richardson constants.
Thermionic emission has been long understood, however, previous efforts to develop suitable electrodes for TEC have been met with only limited success. Some of the first patents describing this energy conversion approach were filed in the 1950's by W. Caldwll (U.S. Pat. No. 2,759,112) and G. Hatsopoulos (U.S. Pat. No. 2,915,652). Both NASA and the Soviet Space Program spent decades of research and billions of dollars in an effort to develop a high-performing TEC device for long duration space flight missions. Some approaches achieved useful DC output currents of ˜20 A/cm2 at an output voltage between 0.5V to 1.5V and operated continuously for 5 or more years. However, these devices had low operational efficiencies (˜20%) and required excessively high operation temperatures (>2000° C.) due to material limitations with work functions in the 4-5 eV range. Effective work functions can be lowered by modifying the surface with cesium; however, the adsorbate techniques typically do not hold up to high fields or long operation times. Thermal sources required for the very high operation temperatures of conventional TEC devices limit the technology.
Examples of conventional thermionic emitters consisted of tungsten, tantalum, and rhenium. The effective work function can be lowered by using thin films of thorium or cesium on the emitter surface, however, these material required co is instant replenishment to prevent depletion due to evaporation. Low work function oxides such as BaO (0.99 eV), SrO (1.27 eV) and CaO (1.77 eV) when deposited as thin films on emitters can yield enhanced thermionic emission but are limited to pulsed or low power applications to prolong their life.
Conventional implementations of thermionic energy converters utilize tungsten cathodes with cesium gas fed into the cathode-anode gap. Tungsten has a relatively high work function requiring high temperatures (in excess of 2000° C.) in order to achieve practical thermionic current densities. The work function can be lowered by “cesiating” the tungsten surface. The effects of the cesium rapidly diminish during operation as the tungsten out gases when heated to operating temperatures. By incorporating cesium gas into the inter-electrode gap, the cesium in the tungsten could be constantly replenished allowing for stable operation. In addition to enhancing the surface chemistry, operation of tungsten cathodes in a cesium vapor environment (rather than a vacuum) has been shown to favorable affect the electron transport from the cathode to the anode.
The high emission currents required to produce necessary output power levels often result in space charge effects. Space charge effects are due to the negatively charged electrons traversing the cathode-anode gap which cancel out a portion of the electric field between the cathode and anode. More electrons present in the gap equates to more of the electric field being canceled, further suppressing the emission current.
This performance limiting effect can be mitigated (or even eliminated) through the presence of positive cesium ions in the cathode-anode gap. One method to introduce these ions is through surface ionization. When a tungsten cathode is heated to temperatures in excess of 1200° C., the cesium atoms that strike it are ionized resulting in positively charged cesium ions. The positive charges present adjacent to the emitter surface cancel out the negative charges of the electrons, reducing the space charge effect. Cesium ions can also be produced by collision of cesium atoms with the thermionically emitted electrons from the cathode. In order for ionization to occur, electron temperatures greater than 2500° C. are required.
In addition, diamond thermionic cathodes in a high pressure (up to 700 mTorr) methane environment greatly increases the emission performance compared to operation in a vacuum environment. However, methane is not a suitable candidate for increasing the performance of a diamond TEC, since operation in methane will result in the accumulation of non-diamond carbonaceous content, akin to soot, preventing the long term operation of such a configuration.
Diamond Thermionic Converters
More recent attempts to capitalize on the potentially high efficiencies of thermionic converters have focused on utilizing diamond electrodes. Diamond has demonstrated exceptionally low work functions, well below 2 eV. Recent patents pertaining to the use of diamond in thermionic generators include I. W. Cox (U.S. Pat. No. 5,981,071), R. J. Nemanich and F. Koeck (U.S. Pat. No. 8,188,456), and C. M. Sung (U.S. 2007/0126312 A1), among others. While employing substitute dopants such as nitrogen, phosphorus, boron, etc. is one technique frequently discussed in the above mentioned patents as a means to improve the performance of diamond films.
Hydrogen places a crucial role in both enhancing electron transport through the bulk of the diamond and also improving the thermionic emission current by lowering the work function.
Cox (U.S. Pat. No. 6,214,651 B1) and Sung (US Patent Publication 2007/0126312 A1) both of which are incorporated by reference herein, teach utilizing hydrogen as a “dopant” in diamond intentionally introducing impurities into an extremely pure semiconductor to change its electrical properties. The use of the term “dopant” implies that hydrogen can be substituted for a carbon atom in the diamond lattice; however, due to hydrogen's single valence electron available for bonding, the term doping likely describes a diamond electrode impregnated with hydrogen, whereby hydrogen lies in the interstitial space between the carbon atoms which is believed to enhance bulk electron transport within diamond which agrees with experimental data indicating exposure of diamond samples to a hydrogen plasma reduces the resistance of the bulk diamond film.
In addition to increasing the bulk conductivity of diamond films, hydrogen has been shown to interact with the diamond surface to form polarized C—H bonds, reducing the electron affinity and in turn, reducing the work function. Exposure of diamond cathodes to a low energy hydrogen plasma prior to testing is known to drastically enhance thermionic emission current relative to as-grown diamond films by four orders of magnitude. Multiple patents teach the surface termination of diamond with hydrogen (i.e. hydrogenation of the diamond surface) such as those by Nemanich et al. (U.S. Pat. No. 8,188,456 B2), Kataoka et al. (US 2015/0075579 A1 and US 2011/0017,253 A1) and Cox (U.S. Pat. No. 6,214,651 B1) and Sung (US 2007/0126312 A1) all of which are incorporated by reference herein.
Hydrogen has consistently been shown to enhance the thermionic emission of diamond films.
Difficulty arises when attempting to utilize hydrogenated diamond electrodes for thermionic generators due to the desorption of hydrogen from the diamond films at elevated temperatures.
Recent studies have shown that hydrogen desorbs from the diamond surface following a predictable time-dependent Arrhenius behavior. When hydrogenated diamond cathodes are heated to temperatures above 600° C., this desorption of the performance-enhancing hydrogen becomes so pronounce that the emission current will strongly deviate from the Richardson equation above and begin to decrease with increasing temperature rather than the predicted increase.
The present invention entails a means to overcome the performance-limiting effect of the desorbtion of the hydrogen from the diamond surface at elevated temperatures of 600° C. or greater.